1. Field of the Invention
The present invention relates to a fluid pressure control valve which is optimally applied to a hydraulic brake system with an anti-skid brake control system generally abbreviated as an "ABS" and/or a traction control system generally abbreviated as a "TCS", and specifically to a fluid pressure controlling spool valve with at least one electromagnetic solenoid which is responsive to the magnitude of a control signal value to suitably adjust the outgoing fluid pressure.
2. Description of the Prior Art
There have been proposed and developed various fluid pressure control valves which can properly vary the incoming fluid pressure to generate a controlled fluid pressure to a fluid pressure-actuated cylinder, such as a vehicle wheel cylinder. For example, Japanese Patent First Publication (Tokkai Heisei) No. 3-65461 discloses a brake fluid pressure control system employing a master cylinder equipped with a hydraulic brake force booster and a plurality of electromagnetic solenoid valves each fluidly disposed between the outlet port of the master cylinder and the inlet-and-outlet port of the wheel cylinder. In the Japanese Patent First Publication No. 3-65461, an anti-skid brake control or a traction control is achieved by switching the respective valve positions of the plural solenoid valves which consist of four three-port/three-position directional control valves mainly contributing to an anti-skid brake control and two two-port/two-position directional control valves mainly contributing to a traction control. The above-mentioned booster device and plural solenoid valves are not integrally formed as a unit but constructed independently of each other. As a consequence, the total weight of the system is relatively heavy and the size of the system is relatively large. It is troublesome to mount such a prior art brake fluid pressure control system in a limited space of the vehicle. In more recent years, there have been proposed brake fluid pressure controlling spool valves which are operable according to an anti-skid brake control. Such a spool valve is designed to adjust the magnitude of the outgoing fluid pressure depending on the axial position of the slidable spool. One such conventional brake fluid pressure controlling spool valve has been disclosed in Japanese Patent First Publication (Tokkai Heisei) No. 3-121969. The conventional fluid pressure controlling spool valve is incorporated into an automotive brake fluid pressure control system to generate a controlled outgoing fluid pressure, i.e., a desired wheel-cylinder pressure by properly adjusting an incoming fluid pressure, i.e., a master-cylinder pressure. The conventional fluid pressure controlling spool valve generally comprises a spool being made of a magnetic material, a return spring normally biasing the spool toward the wheel-cylinder pressure buildup side, and an electromagnetic solenoid attracting the spool toward the wheel-cylinder pressure reduction side. In the above-noted spool valve, the spool has a relatively large diameter to enhance the attraction of the solenoid because the spool itself functions as a solenoid plunger. Therefore, there is a possibility of oil leakage between the outer periphery of the large-diameter spool and the inner periphery of the valve housing. The larger the diameter of the spool becomes, the greater oil leakage becomes. Conversely, the smaller the diameter of the spool becomes, the smaller attracting force of the solenoid becomes. In the previously-noted construction of the conventional spool valve, it is difficult to satisfy both of the above two conditions.